Acoustic wave sensors are utilized in a variety of sensing applications, such as, for example, temperature and/or pressure sensing devices and systems. Acoustic wave devices have been in commercial use for over sixty years. Although the telecommunications industry is the largest user of acoustic wave devices, they are also used for chemical vapor detection. Acoustic wave sensors are so named because they use a mechanical, or acoustic, wave as the sensing mechanism. As the acoustic wave propagates through or on the surface of the material, any changes to the characteristics of the propagation path affect the velocity and/or amplitude of the wave.
Changes in acoustic wave characteristics can be monitored by measuring the frequency or phase characteristics of the sensor and can then be correlated to the corresponding physical quantity or chemical quantity that is being measured. Virtually all acoustic wave devices and sensors utilize a piezoelectric crystal to generate the acoustic wave. Three mechanisms can contribute to acoustic wave sensor response, i.e., mass-loading, visco-elastic and acousto-electric effect. The mass-loading of chemicals alters the frequency, amplitude, and phase and Q value of such sensors. Most acoustic wave chemical detection sensors, for example, rely on the mass sensitivity of the sensor in conjunction with a chemically selective coating that absorbs the vapors of interest resulting in an increased mass loading of the acoustic wave sensor.
Examples of acoustic wave sensors include acoustic wave detection devices, which are utilized to detect the presence of substances, such as chemicals, or environmental conditions such as temperature and pressure. An acoustical or acoustic wave (e.g., SAW/BAW) device acting as a sensor can provide a highly sensitive detection mechanism due to the high sensitivity to surface loading and the low noise, which results from their intrinsic high Q factor. Surface acoustic wave devices are typically fabricated using photolithographic techniques with comb-like interdigital transducers placed on a piezoelectric material. Surface acoustic wave devices may have either a delay line or a resonator configuration. Bulk acoustic wave device are typically fabricated using a vacuum plater, such as those made by CHA, Transat or Saunder. The choice of the electrode materials and the thickness of the electrode are controlled by filament temperature and total heating time. The size and shape of electrodes are defined by proper use of masks.
Surface acoustic wave resonator (SAW-R), surface acoustic wave delay line (SAW-DL), surface transverse wave (STW), bulk acoustic wave (BAW), and acoustic plate mode (APM) all can be utilized in various sensing measurement applications. One of the primary differences between an acoustic wave sensor and a conventional sensor is that an acoustic wave can store energy mechanically. Once such a sensor is supplied with a certain amount of energy (e.g., through RF), the sensor can operate for a time without any active part (e.g., without a power supply or oscillator). This feature makes it possible to implement an acoustic wave in an RF powered passive and wireless sensing application.
Many sensing applications require measurements of multiple parameters. Multiple or groups of sensors are typically employed in these types of applications. Each sensor is dedicated to a particular measurand. In an application such as, for example, oil quality sensing, many factors (e.g., pH, TAN, TBN, viscosity, particulate, lubricity, temperature, flow, pressure, conductivity, humidity, etc.) can contribute to the quality of oil, and thus it is not practical to configure, for example, ten individual sensors in a sensing package. Thus, it would be advantageous to provide a single sensor that can detect multiple parameters. To date, a single sensor for detecting multiple parameters has not been adequately implemented. It is believed that the embodiments disclosed herein overcome the disadvantages of conventional detection systems through the use of a unique surface acoustic sensing arrangement.